Exploring Zosterophyll Relationships within a More Broadly Sampled Character Space: A Focus on Anatomy

Abstract

Premise of research. Zosterophylls are a major constituent of Siluro-Devonian tracheophyte floras worldwide and gave rise to the morphologically diverse lycopsid clade. Despite this pivotal position in the plant tree of life, relationships within the group have remained poorly understood. Furthermore, anatomical data have been undersampled in previous phylogenetic analyses including zosterophylls. With this in mind, we addressed the relationships among zosterophylls using a data set that maximized anatomical characters and employing phylogenetic and phenetic methods.Methodology. A matrix of 40 total characters (12 of which are anatomical) and 19 zosterophyll species with known anatomy, representing 17 genera, was compiled and analyzed using parsimony-constrained phylogenetic methods, clustering, and ordination methods.Pivotal results. Phenetic analyses show sensitivity to taxon sampling and support the placement of Renalia among the zosterophylls but do not support taxonomic inferences strongly congruent with those supported by phylogenetic analyses. Phylogenetic analyses consistently recover two major clades: one lacks internal resolution and comprises the bulk of the zosterophyll taxa included in the analyses; the other clade includes the zosterophyll Ventarura and the lycopsid Sengelia, often accompanied by Discalis and Trichopherophyton. Analyses using subsets of characters (only morphology or anatomy) recover trees that differ from those obtained using the total set of characters (morphology and anatomy).Conclusions. The incongruence between the results of the total character analyses and those using only morphology or only anatomy highlights the importance of broadening the sampling of morphological character space. Because both anatomy and morphology are part of the identity and evolutionary history of a species, the relationships recovered by the inclusion of both morphological and anatomical characters are more likely to reflect natural evolutionary relationships. Breadth of character sampling and not the amount of phylogenetic resolution should be the primary criterion for selecting among alternative hypotheses of relationships.